1 86 PLANT BIOLOGY 



Vaucheria is another alga common in shallow water and on 

 damp soil. The thallus is much branched, but the threads are 

 not divided by cross walls as in spirogyra. The plants are attached 

 by means of colorless root-like organs which are much like the 

 root hairs of the higher plants : these are rhizoids. The chloro- 

 phyll is in the form of grains scattered through the thread. 



Vaucheria has a special mode of asexual reproduction by 

 means of swimming spores or swarm-spores. These are formed 

 singly in a short enlarged lateral branch known as the sporangium. 

 When the sporangium bursts, the entire contents escape, forming 

 a single large swarm-spore, which swims about by means of 

 numerous lashes or cilia on its surface. The swarm spores are so 

 large that they can be seen with the naked eye. After swimming 

 about for some time they come to rest and germinate, producing 

 a new plant. 



The formation of resting-spores of vaucheria is acomplished by 

 means of special organs, oogonia (o, Fig. 268) and antheridia 



(a, Fig. 268). Both of 

 these are specially devel- 

 oped branches from the 

 thallus. The antheridia 

 are nearly cylindrical, and 

 curved toward the oogonia. 

 FIG. 268.- THREAD OF VAUCHERIA WITH The upper part of an an- 

 OOGONIA AND ANTHERIDIA. thendium is cut off by a 



cross wall, and within it 



numerous ciliated sperm-cells are formed. These escape by the 

 ruptured apex of the antheridium. The oogonia are more en- 

 larged than the antheridia, and have a beak-like projection turned 

 a little to one side of the apex. They are separated from the 

 thallus thread by a cross wall, and contain a single large green 

 cell, the egg-cell. The apex of the oogonium is dissolved, and 

 through the opening the sperm-cells enter. Fertilization is thus 

 accomplished. After fertilization the egg-cell becomes invested 

 with a thick wall and is thus converted into a resting-spore, the 

 oospore. 



Fucus. These are rather large specialized algae belonging to the 

 group known as brown seaweeds and found attached by a disk to 

 the rocks of the seashore just below high tide (Fig. 269). They 

 are firm and strong to resist wave action and are so attached as to 

 avoid being washed ashore. They are very abundant algae. In 

 shape the plants are long, branched, and multicellular, with either 

 flat or terete branches. They are olive-brown. Propagation is by 

 the breaking off of the branches. No zoospores are produced, 

 as in many other seaweeds ; and reproduction is wholly sexual. 



